Here's what Roger Ebert has to say:
Wes Anderson's mind must be an exciting place for a story idea to be born. It immediately becomes more than a series of events and is transformed into a world with its own rules, in which everything is driven by emotions and desires as convincing as they are magical. "Moonrise Kingdom" creates such a world and takes place on an island that might as well be ruled by Prospero. It's set in 1965, though it might as well be set at any time.More Here.
On this island no one seems to live except for those involved in the story. There is a lighthouse in which the heroine, Suzy, lives with her family, and a Scout camp where the hero, Sam, stirs restlessly under what seem to him childish restrictions. Sam and Suzy met the previous summer and have been pen pals ever since, plotting a sort of jailbreak from their lives during which they could have an adventure out from under the thumbs of adults, if only for a week.